Heat switches for controlling a flow of heat between thermal stages of a cryostat

ABSTRACT

Heat switches are presented herein for controlling a flow of heat between thermal stages of a cryostat. In one aspect, a heat switch for a cryostat includes a thermal linkage configured to simultaneously contact a first thermal stage and a second thermal stage of the cryostat and define a thermal pathway therebetween. The thermal linkage includes a superconducting element disposed along a portion of the thermal pathway that is capable of transitioning between a superconducting state and a non-superconducting state. A thermal conductivity of the superconducting state is lower than a thermal conductivity of the non-superconducting state. Other types of heat switches are presented, including methods for controlling a flow of heat between thermal stages of a cryostat.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 62/528,831 entitled “Superconducting Heat Switch for ThermallyLinking Temperature Stages of a Cryostat”, filed Jul. 5, 2017, and toU.S. Provisional Application Ser. No. 62/528,859 entitled “MechanicalHeat Switch for Thermally Linking Temperature Stages of a Cryostat”,also filed Jul. 5, 2017. The disclosure of each priority application ishereby incorporated herein by reference.

BACKGROUND

The following description relates to heat switches, and moreparticularly, heat switches for controlling a flow of heat betweenthermal stages of a cryostat.

Cryostats are commonly used to expose devices and samples toenvironments of very low temperature (e.g., T<120 K). Such environmentsare thermally-isolated through insulating walls and are evacuatedenvironments, typically having a pressure in the range of 10⁻³ mbar to10⁻⁹ mbar, thereby allowing the cryostats to operate at stabletemperatures without appreciable thermal losses. Advanced cryostats mayinclude multiple thermal stages, each configured to operate at arespective temperature. Spatial sequences of thermal stages may beconfigured to allow sequentially-adjacent thermal stages to operate atprogressively lower temperatures. The cooldown of a cryostat can be avery time consuming process and structures and methods for reducingcooldown time are desired.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram, shown in perspective, of an examplecryostat having a plurality of thermal stages;

FIG. 2A is a schematic diagram, shown in perspective, of an example heatswitch for a cryostat;

FIG. 2B is a schematic diagram, shown in cross-section, of the exampleheat switch of FIG. 2A;

FIG. 3A is a schematic diagram, shown in perspective, of an example heatswitch for a cryostat;

FIG. 3B is a schematic diagram, shown in cross-section, of the exampleheat switch of FIG. 3A;

FIG. 4A is a schematic diagram, shown in cross-section, of an exampleheat switch in an “off” state and having a thermal linkage that includesa braided body; and

FIG. 4B is a schematic diagram, shown in cross-section, of the exampleheat switch of FIG. 4A, but in which the heat switch is in an “on”state.

DETAILED DESCRIPTION

To achieve very low temperatures, cryostats may employ an outer jacketof cryogenic liquid (e.g., dry-ice in acetone, liquid nitrogen, liquidhelium, etc.) in combination with an inner evacuated chamber thatincorporates one or more “dry” refrigeration systems (e.g., a pulse tuberefrigeration unit, a ³He/⁴He dilution refrigeration unit, etc.).Devices or samples may be disposed on one or more thermal stages, whichin turn, are enclosed within the inner evacuated chamber. The cryostatsmay be configured to thermally-couple such refrigeration systems to oneor more thermal stages, thereby allowing the cryostats to operate theone or more thermal stages at a distribution of temperatures. In manyinstances, the refrigeration systems are thermally-associated with twoor more thermal stages to produce a distribution of progressivelydecreasing temperatures.

For example, a cryostat may include a first thermal stage (or groupingof first stages) and a second thermal stage (or grouping of secondstages). A pulse tube refrigeration unit may extract heat from thesecond thermal stage (or grouping of first stages), which serves as aheat source, and deposit the heat in a second thermal stage (or groupingof second stages), which serves as a heat sink. The second thermal stagethus decreases in temperature relative to the first thermal stage andprovides a basis to create a distribution of progressively decreasingtemperatures (e.g., using a third thermal stage, a fourth thermal stage,and so forth).

To prevent flows of heat from transferring undesirably between thermalstages, cryostats commonly incorporate structural supports of highthermal resistance. The structural supports space the thermal stagesapart to produce a spatial sequence of thermal stages with thecryostats. These structural supports, when combined with the thermalmass of other components in the cryostats, impart thermal lag to thecryostats during operations of cooling and warming. The thermal lag mayalso hinder adjustments to the operating temperatures of individualthermal stages, such as may be required to bring a device or sample to atarget temperature. Mitigating this thermal lag improves set-up timesfor the cryostats and may also improve times necessary to adjust one ormore thermal stages to new operating temperatures.

The embodiments presented herein are directed to heat switches forcryostats that control a flow of heat between thermal stages thereof.The heat switches are operable to selectively transition between an “on”state, where heat flows from a heat source to a heat sink, and an “off”state, where the flow of heat is substantially reduced or eliminated.The heat source and the heat sink may each correspond to one or morethermal stages in a cryostat (e.g., a spatial sequence of thermal stageswithin the cryostat), a device or sample on a thermal stage, or somecombination thereof. Heat flows along a warm-to-cool temperaturegradient and at a magnitude driven by a temperature difference betweenthe heat source and the heat sink.

In one aspect, a heat switch for a cryostat includes a thermal linkageconfigured to simultaneously contact a first thermal stage and a secondthermal stage of the cryostat and define a thermal pathway therebetween.The thermal linkage includes a superconducting element disposed along aportion of the thermal pathway that is capable of transitioning betweena superconducting state and a non-superconducting state. A thermalconductivity of the superconducting state is lower than a thermalconductivity of the non-superconducting state.

In another aspect, a heat switch for a cryostat includes a thermallinkage having a first surface configured to contact a first thermalstage of the cryostat and a second surface configured to contact asecond thermal stage of the cryostat. The thermal linkage is operable toselectively move between a first position, where the thermal linkagesimultaneously contacts the first thermal stage and the second thermalstage and defines a thermal pathway therebetween, and a second position,where the thermal linkage is not contact with at least one of the firstthermal stage and the second thermal stage and the thermal pathway isbroken.

Now referring to FIG. 1, a schematic diagram is presented in perspectiveof an example cryostat 100 having a plurality of thermal stages 102. Theplurality of thermal stages 102 may correspond to radiation shields,thermalization plates, or both. The plurality of thermal stages 102 maybe formed of a material having a high thermal conductivity at cryogenictemperatures, such as below 120 K. For example, the plurality of thermalstages 102 may be formed of a material having a thermal conductivity ofat least 1 W/(m·K) as measured at 4 K. It will be appreciated that suchhigh thermal conductivity allows the plurality of thermal stages 102 tomitigate the development of temperature gradients, thereby maintaining asubstantially uniform temperature across their respective masses.Examples of materials for the plurality of thermal stages includeoxygen-free high conductivity copper and its alloys (e.g., C101 copperalloy).

The cryostat 100 may include any number of thermal stages 102 to supportdevices and samples for cryogenic refrigeration. As a result, thecryostat 100 may position the plurality of thermal stages 102 to definea spatial sequence of thermal stages, such as a in linear sequence or anangular sequence. FIG. 1 depicts four thermal stages 102 in anequally-spaced linear sequence. However, this depiction is not intendedas limiting. In general, the cryostat 100 may include any number andspacing of thermal stages 102 as needed. To achieve such configurations,the cryostat 100 may include one or more structural supports 104 toposition the plurality of thermal stages 102 into the spatial sequenceof thermal stages. The structural supports 104 may be formed of amaterial having a low thermal conductivity at cryogenic temperatures,i.e., less than 0.5 W/(m·K) at or below 50 K, such as a stainless steelalloy or a glass-epoxy laminate of G10 grade. The structural supports104 thus additionally impede a flow of heat between the plurality ofthermal stages 102. As such, the cryostat 100 may include one or morethermal stages dedicated to a specific temperature during operation. Forexample, the cryostat 100 may be configured such that each thermal stageoperates at a progressively decreasing temperature as a depth of thecryostat increases.

The cryostat 100 may also include one or more refrigeration systemsthermally-coupled to each of the thermal stages 102 (not shown). Forexample, the cryostat 100 may include a pulse-tube refrigeration systemcoupled to a second-lowest thermal stage 106 and a ³He/⁴He dilutionrefrigeration system thermally-coupled to a lowest thermal stage 108.The refrigeration systems establish specific operating temperatures forthe thermal stages to which they are respectively thermally-coupled. Indoing so, the refrigeration systems may define a distribution ofoperating temperatures along the spatial sequence of thermal stages.

During operation, the cryostat 100 is cooled and heated to thermallyaffect devices and samples disposed on the plurality of thermal stages102. Such cooling and heating may also be part of processes to maintainthe cryostat 100. However, a rate of cooling and heating may be limitedby the structural supports 104, whose thermal resistance retards heatflows through the cryostat 100 (e.g., between adjacent thermal stages).As such, the cryostat 100 may experience thermal lag during cooling andheating, and sometimes substantially so. It will be appreciated that theheat switches disclosed herein are capable of reducing such thermal lagby controllably establishing heat flows, and in particular selectivelycontrolling a direction of heat, between thermal stages 102 along thespatial sequence of thermal stages.

Furthermore, refrigeration systems commonly have different coolingcapacities (i.e., a rate at which heat may be extracted from athermally-coupled body), and these cooling capacities are often optimalover non-overlapping temperatures (or poorly overlapping). Thesenon-overlapping temperatures may prevent two thermal stages—eachthermally-coupled to a different refrigeration system—from reaching atarget operating temperature at approximately the same time, especiallyif the two thermal stages are thermally-coupled to different types ofrefrigeration systems. The heat switches disclosed herein are capable ofreducing this type of thermal lag by selectively opening a thermalpathway between the two thermal stages, thereby allowing thehigher-capacity refrigeration system to assist the lower-capacityrefrigeration system.

For example, a pulse-tube refrigeration unit may be configured tooptimally extract heat at temperatures to about 4 K and a ³He/⁴Hedilution refrigeration unit may be configured to optimally extract heatat temperatures below 1 K. The pulse-tube refrigeration unit may bethermally-coupled to the lowest thermal stage 108 and the second-lowestthermal stage 106 and used to pre-cool these two thermal stages to about4 K. The ³He/⁴He dilution refrigeration unit may be thermally-coupledthe lowest thermal stage 108 and used to cool the lowest thermal stage108 to temperatures below 1 K. However, cooling of the lowest thermalstage 108 by ³He/⁴He dilution refrigeration unit occurs afterpre-cooling of the lowest thermal stage 108 and the second-lowestthermal stage 106 by the pulse-tube refrigeration unit.

Thermal coupling between the second-lowest thermal stage 106 and thelowest thermal stage 108 influences a rate of heat transfer between thetwo thermal stages during cooling. Thermal coupling based on a highthermal conductivity pathway will increase the rate of heat transfer anddecrease an overall time required to cool the cryostat. For example, ifthe second-lowest thermal stage 106 is thermally-coupled to thepulse-tube refrigeration unit through a high thermal conductivitypathway, the second-lowest thermal stage 106 may cool to 4 K at anacceptable rate during operation. However, if the cryostat relies solelyon the structural supports 104 for thermal coupling to the lowestthermal stage 108, the overall time required to cool the lowest thermalstage 108 to 4K is not optimal as the structural supports 104 are formedof low thermal conductivity material. As a result, the lowest thermalstage 108, which is thermally-coupled to the ³He/⁴He dilutionrefrigeration unit, reaches a target temperature later than thesecond-lowest thermal stage 106, which is thermally-coupled to thepulse-tube refrigeration unit. (The ³He/⁴He dilution refrigeration unittypically has a lower cooling capacity above 4 K than the pulse tuberefrigeration unit.) This time difference is referred to herein as thethermal lag.

The heat switches disclosed herein may reduce this thermal lag byestablishing a temporary thermal “short circuit” between the lowestthermal stage 108 and the second-lowest thermal stage 106, therebyallowing the pulse-tube refrigeration unit to more efficiently cool downthe cryostat prior to starting the ³He/⁴He dilution refrigeration unit.

Now referring to FIG. 2A, a schematic diagram is shown in perspective ofan example heat switch 200 for a cryostat. A cross-sectional view of theexample heat switch 200 is shown in FIG. 2B. The heat switch 200includes a thermal linkage 202 configured to simultaneously contact afirst thermal stage 204 and a second thermal stage 206 of the cryostatand define a thermal pathway 208 therebetween (see dashed line 208).FIGS. 2A & 2B depict the thermal pathway 208 as a single, straightpathway. However, this depiction is not intended as limiting. Thethermal pathway 208 may be non-straight (e.g., curved, segmented, etc.)and include multiple pathways therein. The multiple pathways may divergeor converge along the thermal linkage 202 as set by characteristics sucha shape of the thermal linkage 202, a number of components that definethe thermal linkage 202, one or more thermal properties of thecomponents, interfaces between the components, and so forth.

The thermal linkage 202 includes a superconducting element 210 disposedalong a portion of the thermal pathway 208 and capable of transitioningbetween a superconducting state and a non-superconducting state. Athermal conductivity of the superconducting state is lower than athermal conductivity of the non-superconducting state. Thesuperconducting element 210 is formed of a superconducting materialcapable of entering a superconducting state below a superconductingcritical temperature, such as a Type-I or Type-II superconductor.Examples of the superconducting material include lead and its alloys,indium and its alloys, tin and its alloys, thallium and its alloys,palladium and its alloys, and niobium and its alloys. For example, thesuperconducting element 210 may be formed of lead, which has a highthermal conductivity in a temperature range from 4 K to 10 K and a lowthermal conductivity (i.e., two orders of magnitude lower) in amilli-Kelvin temperature range below its superconducting criticaltemperature (i.e., T_(sc)=7.19 K).

In some instances, the thermal conductivity of the superconducting stateis no greater than one-third of the thermal conductivity of thenon-superconducting state at 300 K. In some instances, the thermalconductivity of the superconducting state is no greater than one-fifthof the thermal conductivity of the non-superconducting state at 300 K.In some instances, the thermal conductivity of the superconducting stateis no greater than one-tenth of the thermal conductivity of thenon-superconducting state at 300 K. In some instances, the thermalconductivity of the superconducting state is no greater than one-twelfthof the thermal conductivity of the non-superconducting state at 300 K.

In some implementations, the superconducting element 210 includes alayer. The layer may be disposed at any position along the thermalpathway 208. For example, as shown in FIGS. 2A & 2B, the layer may bedisposed at a midpoint of the thermal linkage 202. In someimplementations, the superconducting element 210 includes a layerconfigured to contact a surface of the first thermal stage 204 or asurface of the second thermal stage 206. In these implementations, thelayer may be disposed at an interface between the thermal linkage 202and a thermal stage when the heat switch 200 is incorporated into thecryostat.

Although FIGS. 2A & 2B illustrate only one instance of thesuperconducting element 210 this illustration is not intended aslimiting. In general, the thermal linkage 202 may include any number ofsuperconducting elements and at any position within the thermal linkage202 along the thermal pathway 208. Moreover, the superconducting element208 may have any shape as necessary to interrupt a continuity of thethermal pathway 208.

The thermal linkage 202 may include a thermally-conductive element 212having a thermal conductivity of at least 1 W/(m·K) at 4 K. For example,the thermally-conductive element 212 may be formed of oxygen-free highconductivity copper and its alloys (e.g., C101 copper alloy) with a highresidual resistivity ratio (RRR). The thermally-conductive element 212may be formed into a rod, a tube, a cylinder, a braid or other wovenarticle, a sheet, or a truss. Other forms are possible, includingcombinations of forms. FIGS. 2A & 2B depict two thermally-conductiveelements 212. However, this depiction is for purposes of illustrationonly. The heat switch 200 may optionally include an electric heaterconfigured to supply heat to the thermally-conductive element. Theelectric heater may be operable to alter a thermal conductivity of thethermally-conductive element 212 by altering a temperature of thethermally-conductive element 212 (or some portion thereof). In someinstances, such as shown in FIGS. 2A & 2B, the thermally-conductiveelement 212 includes a cylindrical body (e.g., a rod, a tube, acylinder, etc.). In some instances, the thermally-conductive element 212includes a braided body (e.g., a cloth, a fabric, a braid, etc.).

In some implementations, the heat switch 200 includes a fastener 214configured to couple the thermal linkage 202 to the first thermal stage204 or the second thermal stage 206. The fastener 214 is operable toapply a clamping force to the thermal linkage 202, thereby improvingcontact between the thermal linkage 202 and a corresponding thermalstage. This improved contact may also improve a conduction of heatacross an interface between the thermal linkage 202 and thecorresponding stage. The fastener 214 may be configured to allowselective coupling and de-coupling of the thermal linkage 202 from thethermal stages, such as common to a threaded fastener. For example, asshown in FIGS. 2A & 2B, the heat switch 200 may include a first socketcap screw to couple the thermal linkage 202 to the first thermal stage204 and a second socket cap screw to couple the thermal linkage 202 tothe second thermal stage 206. However, other types of fasteners arepossible (e.g., latches, brads, pins, etc.).

In some implementations, shown in FIGS. 2A & 2B, the heat switch 200includes an electric heater 216 configured to supply heat to thesuperconducting element 210. The electric heater 216 may be in directcontact with the superconducting element 210 and coupled thereto througha fastener (e.g., a screw, a rivet, a clamp, etc.), a weld joint, or abraze joint. Other means of coupling are possible. The electric heater216 is operable to heat a superconducting material forming thesuperconducting element 210 above its superconducting criticaltemperature, thus inducing the superconducting element 210 to exit thesuperconducting state and enter the non-superconducting state.

In some implementations, the heat switch 200 includes an electromagnetconfigured to apply a magnetic field to the superconducting element 210.Like the electric heater 216, the electromagnet may be disposedproximate the superconducting element 210 (e.g., adjacent,wrapped-around, etc.). The electromagnet may optionally includesuperconducting wire to minimize heat produced by electrical currentgenerating the magnetic field. In such implementations, theelectromagnet is operable to apply the magnetic field at a magnitude ator above a superconducting critical magnetic field of a superconductingmaterial forming the superconducting element 210. In response, thesuperconducting element 210 exits the superconducting state and entersthe non-superconducting state.

It will be appreciated that the heat switch 200 may be incorporatedwithin a cryostat, such as shown in FIGS. 2A & 2B. The heat switch 200may then include the first thermal stage 204 and the second thermalstage 206 of the cryostat. When incorporated within the cryostat, thethermal linkage 202 is in simultaneous contact with the first thermalstage 204 and the second thermal stage 206. The first thermal stage 204may be configured to operate at a first temperature, and the secondthermal stage 206 may be configured to operate at a second temperaturelower than the first temperature. In some instances, second thermalstage 206 is cooled to a temperature no greater than 10 K. In someinstances, the second thermal stage 206 is cooled to a temperature nogreater than 10 K and the first thermal stage 204 is cooled to atemperature no greater than 75 K.

The heat switch 200 may also be incorporated into the cryostat to coupleany combination of two or more thermal stages. As such, the firstthermal stage 204 and the second thermal stage 206 may correspond toadjacent thermal stages or thermal stages separated by one or more otherthermal stages. The first thermal stage 204 and the second thermal stage206 may also correspond to two thermal stages in a grouping of three ormore thermal stages coupled by the heat switch 200. In some instances,such as shown in FIGS. 2A & 2B, the cryostat includes a spatial sequenceof thermal stages and the first thermal stage 204 and the second thermalstage 206 are adjacent each other in the spatial sequence of thermalstages. In other instances, the cryostat comprises a spatial sequence ofthermal stages and the first thermal stage 204 and the second thermalstage 206 are separated by at least one thermal stage in the spatialsequence of thermal stages.

In operation, the heat switch 200 controls a flow of heat from a heatsource to a heat sink. To do so, the heat switch 200 selectivelytransitions between an “on” state, where heat flows from a heat sourceto a heat sink, and an “off” state, where the flow of heat issubstantially reduced or eliminated. The heat source and the heat sinkmay each correspond to one or more thermal stages in a cryostat (e.g., aspatial sequence of thermal stages within the cryostat)), a device orsample on a thermal stage, or some combination thereof. The flow of heattraverses along a warm-to-cool temperature gradient and at a magnitudedriven by a temperature difference between the heat source and the heatsink.

During operation of the cryostat, the heat switch 200 may be cooled tocryogenic temperatures (e.g., T<120 K) below the superconductingcritical temperature associated with the superconducting element 210. Assuch, the superconducting element 210 enters the superconducting stateand decreases in thermal conductivity. This decrease imparts thermalresistance along the thermal pathway 208, thereby transitioning the heatswitch 200 into the “off” state. For heat to traverse the thermallinkage 202, the heat switch 200 may be selectively activated into the“on” state by energizing the electric heater 216. The electric heater216 applies heat to the superconducting element 210 to raise itstemperature above the superconducting critical temperature. In response,the superconducting element 210 enters the non-superconducting state andincreases in thermal conductivity. This increase removes thermalresistance along the thermal pathway 208 and thus transitions the heatswitch 200 into the “on” state. It will be appreciated that, because theheat switch 200 operates without being physically moved, the heat switch200 may be considered a “passive” heat switch.

In some variations, the heat switch 200 may use an electromagnet totransition the superconducting element 210 between the superconductingstate and the non-superconducting state. In these variations, theelectromagnet may supplement the electric heater 216, or alternatively,replace the electric heater 216. During operation, the electromagnetapplies a magnetic field to the superconducting element 210 at amagnitude equal to or greater than the superconducting critical magneticfield associated with the superconducting element 210. In response, thesuperconducting element 210 exits the superconducting state and entersthe non-superconducting state. De-energizing the electromagnet removesthe magnetic field, allowing the superconducting element 210 to returnto the superconducting state. It will be appreciated that theelectromagnet provides a non-thermal means to transition thesuperconducting element 210 between the superconducting state and thenon-superconducting state. As such, heat from the electric heater 216may be reduced or eliminated, thereby reducing a thermal load that mustotherwise be processes by refrigeration systems of the cryostat.

Selective activation of the heat switch 200 allows a flow of heat to betransferred between the first thermal stage 204 and the second thermalstage 206. For example, during cooling of the cryostat, the secondthermal stage 206 may thermally lag (in temperature) the first thermalstage 204. This thermal lag may result from thermally-insulatingcharacteristics of structural supports between the first and secondthermal stages 204, 206. Cooling of the second thermal stage 206 may beaccelerated by activating the heat switch 200 into “on” state. Thisactivation may allow refrigeration systems associated with the first andsecond thermal stages 204, 206 to operate in tandem, thereby improving aheat extraction rate from the second thermal stage 206. In anotherexample, the heat switch 200 may also be selectively activated duringheating of the cryostat, such as when the first and second thermalstages 204, 206 of the cryostat are brought to room temperature fromcryogenic temperatures. During heating, the second thermal stage 206 mayalso thermally lag (in temperature) the first thermal stage 204. Thisthermal lag may be mitigated by activating the heat switch 200 into “on”state, thereby increase a supply rate of heat to the second thermalstage 206. In yet another example, the heat switch 200 may be activatedto adjust an operating temperature of the first thermal stage 204, thesecond thermal stage 206, or both, to bring a device or sample to atarget temperature.

According to an implementation, a method for controlling a flow of heatbetween thermal stages of a cryostat includes contacting a thermallinkage to a first thermal stage and a second thermal stage of thecryostat to establish a thermal pathway therebetween. The thermallinkage includes a superconducting element disposed along a portion ofthe thermal pathway. The method also includes altering a firsttemperature of the first thermal stage and altering a second temperatureof the second thermal stage. The method additionally includesselectively transitioning the superconducting element between asuperconducting state, where a flow of heat between the first thermalstage and the second thermal stage is decreased, and anon-superconducting state, where a flow of heat between the firstthermal stage and the second thermal stage is increased. In someinstances, the cryostat includes a spatial sequence of thermal stagesand the first thermal stage and the second thermal stage are adjacenteach other in the spatial sequence of thermal stages. In otherinstances, the cryostat includes a spatial sequence of thermal stagesand the first thermal stage and the second thermal stage are separatedby at least one thermal stage in the spatial sequence of thermal stages.

In some implementations, selectively transitioning the superconductingelement includes cooling the superconducting element at or below asuperconducting critical temperature to enter the superconducting stateand heating the superconducting element above the superconductingcritical temperature to enter the non-superconducting state. In furtherimplementations, selectively transitioning the superconducting elementmay also include applying a magnetic field to the superconductingelement. A magnitude of the applied magnetic field may be increased toat least a superconducting critical magnetic field, thereby inducing thesuperconducting element to enter the non-superconducting state. Themagnitude of the applied magnetic field may also be decreased to belowthe superconducting critical magnetic field, thereby inducing thesuperconducting element to enter the superconducting state.

In some implementations, altering the first temperature of the firstthermal stage includes cooling the first thermal stage and altering thesecond temperature of the second thermal stage includes cooling thesecond thermal stage. In these implementations, transitioning thesuperconducting element includes transferring heat from the secondthermal stage to the first thermal stage when the superconductingelement is in the non-superconducting state. In some instances, thesecond thermal stage is cooled to a temperature no greater than 10 K. Inother instances, the second thermal stage is cooled to a temperature nogreater than 10 K and the first thermal stage is cooled to a temperatureno greater than 75 K.

In some implementations, altering the first temperature of the firstthermal stage includes heating the first thermal stage and altering thesecond temperature of the second thermal stage includes heating thesecond thermal stage. In these implementations, transitioning thesuperconducting element includes transferring heat from the firstthermal stage to the second thermal stage when the superconductingelement is in the non-superconducting state. In some instances, thesecond thermal stage is heated from a temperature no greater than 10 K.In other instances, the second thermal stage is heated from atemperature no greater than 10 K and the first thermal stage is heatedfrom a temperature no greater than 75 K.

Now referring to FIG. 3A, a schematic diagram is shown in perspective ofan example heat switch 300 for a cryostat. A cross-sectional view of theexample heat switch 300 is shown in FIG. 3B. The heat switch 300includes a thermal linkage 302 having a first surface 304 configured tocontact a first thermal stage 306 of the cryostat and a second surface308 configured to contact a second thermal stage 310 of the cryostat.The thermal linkage 302 is operable to selectively move between a firstposition, where the thermal linkage simultaneously contacts the firstthermal stage 306 and the second thermal stage 310 and defines a thermalpathway 312 therebetween, and a second position, where the thermallinkage 302 is not contact with at least one of the first thermal stage306 and the second thermal stage 310 and the thermal pathway 312 isbroken. FIGS. 3A & 3B depict the thermal linkage 302 in the firstposition. However, this depiction is for purposes of illustration only.Other positions of the thermal linkage 302 are possible.

FIGS. 3A & 3B depict the thermal pathway 312 as a single, straightpathway. However, this depiction is not intended as limiting. Thethermal pathway 312 may be non-straight (e.g., curved, segmented, etc.)and include multiple pathways therein. The multiple pathways may divergeor converge along the thermal linkage 302 as set by characteristics sucha shape of the thermal linkage 302, a number of components that definethe thermal linkage 302, one or more thermal properties of thecomponents, interfaces between the components, and so forth.

In some implementations, the thermal linkage 302 is formed of a materialhaving a thermal conductivity of at least 1 W/(m·K) at 4 K. For example,the thermal linkage 302 may be formed of oxygen-free high conductivitycopper and its alloys (e.g., C101 copper alloy) with a high residualresistivity ratio (RRR). The thermal linkage 302 may be formed into arod, a tube, a cylinder, a braid or other woven article, a sheet, or atruss. Other forms are possible, including combinations of forms. Theheat switch 300 may optionally include an electric heater 314 configuredto supply heat to the thermally-conductive element. The electric heater314 may be operable to alter a thermal conductivity of the thermallinkage 302 by altering a temperature of one or more portions thereon.In some instances, such as shown in FIGS. 3A & 3B, the material includesa cylindrical body (e.g., a rod, a tube, a cylinder, etc.). In someinstances, the material includes a braided body (e.g., a cloth, afabric, a braid, etc.).

In some implementations, the thermal linkage 302 is configured to movelinearly between the first position and the second position (see dashedarrow 316). In other implementations, the thermal linkage is configuredto rotate between the first position and the second position. In someimplementations, the thermal linkage 302 is configured to both movelinearly and rotate between the first position and the second position.

In some implementations, the heat switch 300 is configured such that thethermal linkage 302 is moved manually by an operator between the firstposition and the second position. In some implementations, the heatswitch 300 includes an actuator coupled to the thermal linkage 302 andoperable to move the thermal linkage 302 between the first position andthe second position. For example, the heat switch 300 may include anelectric motor coupled to a shaft of the thermal linkage 302.

In some embodiments, the heat switch 300 includes an actuator coupled tothe thermal linkage 302 and also includes a thermally-insulating elementcoupling the actuator to the thermal linkage 302. Thethermally-insulating element is operable to impede flows of heat fromthe actuator to the thermal linkage 302 (and vice versa). This behaviormay reduce a rate of heat transfer from upper thermal stages of thecryostat to colder, lower thermal stages of the cryostat. The behaviormay also reduce a rate of heat transfer from an outer vacuum chamberencapsulating the cryostat to the colder, lower thermal stages of thecryostat. The thermally-insulating element may be formed of a materialhaving a thermal conductivity less than 0.5 W/(m·K) at or below 50 K.For example, thermally-insulating element may be formed of a glass-epoxylaminate of G10 grade per specifications of the National ElectronicManufacturers Association (NEMA). The thermally-insulating element 302may be formed into a rod, a tube, a cylinder, a braid, or other wovenarticle. Other forms are possible, including combinations of forms. Insome instances, the material includes a cylindrical body (e.g., a rod, atube, a cylinder, etc.). In some instances, the material includes abraided body (e.g., a cloth, a fabric, a braid, etc.).

It will be understood that the heat switch 300 may be incorporatedwithin a cryostat, such as shown in FIGS. 3A &3. The heat switch 300 maythen include the first thermal stage 306 and the second thermal stage310 of the cryostat. The first thermal stage 306 may be configured tooperate at a first temperature, and the second thermal stage 310 may beconfigured to operate at a second temperature lower than the firsttemperature. In some instances, second thermal stage 310 is cooled to atemperature no greater than 10 K. In some instances, the second thermalstage 310 is cooled to a temperature no greater than 10 K and the firstthermal stage 306 is cooled to a temperature no greater than 75 K.

The heat switch 300 may also be incorporated into the cryostat to coupleany combination of two or more thermal stages. As such, the firstthermal stage 306 and the second thermal stage 310 may correspond toadjacent thermal stages or thermal stages separated by one or more otherthermal stages. The first thermal stage 306 and the second thermal stage310 may also correspond to two thermal stages in a grouping of three ormore thermal stages coupled by the heat switch 300. In some instances,such as shown in FIGS. 3A & 3B, the cryostat includes a spatial sequenceof thermal stages and the first thermal stage 306 and the second thermalstage 310 are adjacent each other in the spatial sequence of thermalstages. In other instances, the cryostat comprises a spatial sequence ofthermal stages and the first thermal stage 306 and the second thermalstage 310 are separated by at least one thermal stage in the spatialsequence of thermal stages.

In operation, the heat switch 300 controls a flow of heat from a heatsource to a heat sink. To do so, the heat switch 300 selectivelytransitions between an “on” state, where heat flows from a heat sourceto a heat sink, and an “off” state, where the flow of heat issubstantially reduced or eliminated. The heat source and the heat sinkmay each correspond to one or more thermal stages in a cryostat (e.g., aspatial sequence of thermal stages within the cryostat), a device orsample on a thermal stage, or some combination thereof. The flow of heattraverses along a warm-to-cool temperature gradient and at a magnitudedriven by a temperature difference between the heat source and the heatsink.

During operation of the cryostat, the heat switch 300 may be cooled tocryogenic temperatures (e.g., T<120 K) below the superconductingcritical temperature associated with the superconducting element 210. Totransition into the “on” state, the heat switch 300 may move into thefirst position, thereby establishing the thermal pathway 312 between thefirst thermal stage 306 and the second thermal stage 310. Such movementmay occur in response to manual operation by an operator or via anactuator. In particular, the first surface 304 of the thermal linkage302 contacts the first thermal stage 306 and the second surface 308contacts the second thermal stage 310. Due to the high thermalconductivity associated with the thermal linkage 302, heat may flow fromthe first thermal stage 306 to the second thermal stage 310 (or viceversa).

To transition into the “off” state, the heat switch 300 may move intothe second position, where at least one of the first surface 304 and thesecond surface 308 is not in contact with, respectively, the firstthermal stage 306 or the second thermal stage 310. Such movement mayoccur in response to manual operation by an operator or via an actuator.Because the thermal pathway 312 is broken, heat ceases to flow betweenthe first thermal stage 306 and the second thermal stage 310. It will beappreciated that, because the heat switch 200 operates being physicallymoved, the heat switch 200 may be considered an “active” heat switch.

Selective activation of the heat switch 300 allows a flow of heat to betransferred between the first thermal stage 306 and the second thermalstage 310. For example, during cooling of the cryostat, the secondthermal stage 310 may thermally lag (in temperature) the first thermalstage 306. This thermal lag may result from thermally-insulatingcharacteristics of structural supports between the first and secondthermal stages 306, 310. Cooling of the second thermal stage 310 may beaccelerated by activating the heat switch 300 into “on” state. Thisactivation may allow refrigeration systems associated with the first andsecond thermal stages 306, 310 to operate in tandem, thereby improving aheat extraction rate from the second thermal stage 310. In anotherexample, the heat switch 300 may also be selectively activated duringheating of the cryostat, such as when the first and second thermalstages 306, 310 of the cryostat are brought to room temperature fromcryogenic temperatures. During heating, the second thermal stage 310 mayalso thermally lag (in temperature) the first thermal stage 306. Thisthermal lag may be mitigated by activating the heat switch 300 into “on”state, thereby increase a supply rate of heat to the second thermalstage 310. In yet another example, the heat switch 300 may be activatedto adjust an operating temperature of the first thermal stage 306, thesecond thermal stage 310, or both, to bring a device or sample to atarget temperature.

In some implementations, a thermally-conductive element is disposedbetween the first surface 304 of the thermal linkage 302 and the firstthermal stage 306, disposed between the second surface 308 of thethermal linkage 302 and the second thermal stage 310, or both. Thethermally-conductive element is operable to improve heat transfer acrossan interface defined by mating surfaces of the thermal linkage 302 and acorresponding thermal stage. The thermally-conductive element may beformed of a material having a thermal conductivity of at least 1 W/(m·K)at 4 K, such as oxygen-free high conductivity copper and its alloys(e.g., C101 copper alloy) with a high residual resistivity ratio (RRR).In some instances, the thermally-conductive element includes a braidedbody (e.g., a cloth, a fabric, a braid, a loop, etc.).

Now referring to FIG. 4A, a schematic diagram is presented incross-section of a heat switch 400 in an “off” state and having athermal linkage 402 that includes a braided body. FIG. 4B presents theheat switch 400 of FIG. 4A, but in an “on” state. The heat switch 400includes a thermally conductive element 402 that is thermally-connectedto a first thermal stage 406 using a thermally conductive braid 412. Thethermally-conductive braid 412 is attached at both ends bythermally-conductive couplings 404, 414 to the first thermal stage androd, respectively. The thermally-conductive element 402 can movelinearly as well as rotate. The heat switch 400 depicted in FIG. 4A isin the “off” state as the thermally conductive element 402 is notfastened into the threaded, thermal anchor 410, which is thermallyconnected to the second thermal stage 408. This configuration does notcreate a thermal pathway between the first thermal stage 406 and thesecond thermal stage 408. The heat switch 400 depicted in FIG. 4B is inthe “on” state as the thermally conductive element 402 is fastened intothe threaded, thermal anchor 410. When fastening the thermallyconductive element 402 to the threaded, thermal anchor 410, a rotationalmotion will be generated. This rotational motion winds thethermally-conductive braid around the thermally conductive element 402.The use of a long thermally conductive braid 412 will allow fastening ofthe thermally conductive element 402 to the threaded thermal anchor 410.The “on” state creates a thermal short between the first thermal stage406 and the second thermal stage 408.

According to an implementation, a method for controlling a flow of heatbetween thermal stages of a cryostat includes altering a firsttemperature of a first thermal stage of the cryostat and altering asecond temperature of a second thermal stage of the cryostat. The methodalso includes selectively moving a thermal linkage between a firstposition, where the thermal linkage simultaneously contacts the firstthermal stage and the second thermal stage and defines a thermal pathwaytherebetween, and a second position, where the thermal linkage is notcontact with at least one of the first thermal stage and the secondthermal stage and the thermal pathway is broken. In some instances, thecryostat includes a spatial sequence of thermal stages and the firstthermal stage and the second thermal stage are adjacent each other inthe spatial sequence of thermal stages. In other instances, the cryostatincludes a spatial sequence of thermal stages and the first thermalstage and the second thermal stage are separated by at least one thermalstage in the spatial sequence of thermal stages.

In some implementations, selectively moving the thermal linkage includesmoving the thermal linkage linearly between the first position and thesecond position. In some implementations, selectively moving the thermallinkage includes rotating the thermal linkage between the first positionand the second position.

In some implementations, altering the first temperature of the firstthermal stage includes cooling the first thermal stage and altering thesecond temperature of the second thermal stage includes cooling thesecond thermal stage. In these implementations, selectively moving thethermal linkage includes transferring heat from the second thermal stageto the first thermal stage when the thermal linkage is in the firstposition. In some instances, the second thermal stage is cooled to atemperature no greater than 10 K. In other instances, the second thermalstage is cooled to a temperature no greater than 10 K and the firstthermal stage is cooled to a temperature no greater than 75 K.

In some implementations, altering the first temperature of the firstthermal stage includes heating the first thermal stage and altering thesecond temperature of the second thermal stage includes heating thesecond thermal stage. In these implementations, selectively moving thethermal linkage includes transferring heat from the first thermal stageto the second thermal stage when the thermal linkage is in the firstposition. In some instances, the second thermal stage is heated from atemperature no greater than 10 K. In other instances, the second thermalstage is heated from a temperature no greater than 10 K and the firstthermal stage is heated from a temperature no greater than 75 K.

It will be understood that implementations of the heat switchesdisclosed herein are not limited to a single instance per cryostat, asshown in FIGS. 2A-3B, but may also include multiple instances percryostat. Moreover, any combination of active heat switches and passiveheat switches may be used. For example, a cryostat may include a singleactive heat switch that defines a thermal pathway between two or morethermal stages. In another example, a cryostat may include a passiveheat switch that defines a thermal pathway between a first thermal stageand a second thermal stage and an active heat switch that defines athermal pathway between a third thermal stage and a fourth thermalstage. The second thermal stage and the third thermal stage may beshared in common. Other numbers, type combinations, and thermal-pathconfigurations are possible.

Implementations of the heat switches described herein and methods forcontrolling a flow of heat between thermal stages of a cryostat of mayalso be described by the following non-limiting examples:

-   Example 1 A heat switch for a cryostat, comprising:    -   a thermal linkage configured to simultaneously contact a first        thermal stage and a second thermal stage of the cryostat and        define a thermal pathway therebetween, the thermal linkage        comprising:        -   a superconducting element disposed along a portion of the            thermal pathway and capable of transitioning between a            superconducting state and a non-superconducting state, and        -   wherein a thermal conductivity of the superconducting state            is lower than a thermal conductivity of the            non-superconducting state.-   Example 2 The heat switch of example 1, wherein the thermal    conductivity of the superconducting state is no greater than    one-tenth of the thermal conductivity of the non-superconducting    state at 300 K.-   Example 3 The heat switch of example 1 or 2, wherein the thermal    linkage comprises a thermally-conductive element having a thermal    conductivity of at least 1 W/(m·K) at 4 K.-   Example 4. The heat switch of example 3, wherein the    thermally-conductive element comprises a cylindrical body.-   Example 5. The heat switch of example 3 or 4, wherein the    thermally-conductive element comprises a braided body.-   Example 6. The heat switch of example 3 or any one of examples 4-5,    comprising an electric heater configured to supply heat to the    thermally-conductive element.-   Example 7. The heat switch of claim 1 or any one of examples 2-6,    comprising a fastener configured to couple the thermal linkage to    the first thermal stage or the second thermal stage.-   Example 8. The heat switch of example 1 or any one of examples 2-7,    wherein the superconducting element comprises a layer.-   Example 9. The heat switch of example 1 or any one of examples 2-7,    wherein the superconducting element comprises a layer configured to    contact a surface of the first thermal stage or a surface of the    second thermal stage.-   Example 10. The heat switch of example 1 or any one of examples 2-9,    comprising an electric heater configured to supply heat to the    superconducting element.-   Example 11. The heat switch of example 1 or any one of examples    2-10, comprising an electromagnet configured to apply a magnetic    field to the superconducting element.-   Example 12. The heat switch of example 1 or any one of examples    2-11, comprising:    -   the first thermal stage and the second thermal stage of the        cryostat, the thermal linkage in simultaneous contact with the        first thermal stage and the second thermal stage;    -   wherein the first thermal stage is configured to operate at a        first temperature; and    -   wherein the second thermal stage is configured to operate at a        second temperature lower than the first temperature.-   Example 13. The heat switch of example 12, wherein the cryostat    comprises a spatial sequence of thermal stages and the first thermal    stage and the second thermal stage are adjacent each other in the    spatial sequence of thermal stages.-   Example 14. The heat switch of example 12, wherein the cryostat    comprises a spatial sequence of thermal stages and the first thermal    stage and the second thermal stage are separated by at least one    thermal stage in the spatial sequence of thermal stages.-   Example 15. The heat switch of example 12 or any one of examples    13-14, wherein the second temperature is no greater than 10 K.-   Example 16. The heat switch of example 12 or any one of examples    13-14, wherein the second temperature is no greater than 10 K and    the first temperature is no greater than 75 K.-   Example 17. A method for controlling a flow of heat between thermal    stages of a cryostat, the method comprising:    -   contacting a thermal linkage to a first thermal stage and a        second thermal stage of the cryostat to establish a thermal        pathway therebetween, the thermal linkage comprising a        superconducting element disposed along a portion of the thermal        pathway;    -   altering a first temperature of the first thermal stage;    -   altering a second temperature of the second thermal stage; and    -   selectively transitioning the superconducting element between a        superconducting state, where a flow of heat between the first        thermal stage and the second thermal stage is decreased, and a        non-superconducting state, where a flow of heat between the        first thermal stage and the second thermal stage is increased.-   Example 18. The method of example 17, wherein selectively    transitioning the superconducting element comprises:    -   cooling the superconducting element at or below a        superconducting critical temperature to enter the        superconducting state; and    -   heating the superconducting element above the superconducting        critical temperature to enter the non-superconducting state.-   Example 19. The method of example 18, wherein selectively    transitioning the superconducting element comprises:    -   applying a magnetic field to the superconducting element;    -   increasing a magnitude of the applied magnetic field to at least        a superconducting critical magnetic field, thereby inducing the        superconducting element to enter the non-superconducting state;        and    -   decreasing the magnitude of the applied magnetic field to below        the superconducting critical magnetic field, thereby inducing        the superconducting element to enter the superconducting state.-   Example 20. The method of example 17 or any one of examples 18-19,    -   wherein altering the first temperature of the first thermal        stage comprises cooling the first thermal stage;    -   wherein altering the second temperature of the second thermal        stage comprises cooling the second thermal stage; and    -   wherein transitioning the superconducting element comprises        transferring heat from the second thermal stage to the first        thermal stage when the superconducting element is in the        non-superconducting state.-   Example 21. The method of example 20, wherein second thermal stage    is cooled to a temperature no greater than 10 K.-   Example 22. The method of example 20, wherein the second thermal    stage is cooled to a temperature no greater than 10 K and the first    thermal stage is cooled to a temperature no greater than 75 K.-   Example 23. The method of example 17 or any one of examples 18-19,    -   wherein altering the first temperature of the first thermal        stage comprises heating the first thermal stage;    -   wherein altering the second temperature of the second thermal        stage comprises heating the second thermal stage; and    -   wherein transitioning the superconducting element comprises        transferring heat from the first thermal stage to the second        thermal stage when the superconducting element is in the        non-superconducting state.-   Example 24. The method of example 23, wherein the second thermal    stage is heated from a temperature no greater than 10 K.-   Example 25. The method of example 23, wherein the second thermal    stage is heated from a temperature no greater than 10 K and the    first thermal stage is heated from a temperature no greater than 75    K.-   Example 26. The method of claim 17 or any one of examples 18-25,    wherein the cryostat comprises a spatial sequence of thermal stages    and the first thermal stage and the second thermal stage are    adjacent each other in the spatial sequence of thermal stages.-   Example 27. The method of example 17 or any one of examples 18-25,    wherein the cryostat comprises a spatial sequence of thermal stages    and the first thermal stage and the second thermal stage are    separated by at least one thermal stage in the spatial sequence of    thermal stages.-   Example 28. A heat switch for a cryostat, comprising:    -   a thermal linkage, comprising:        -   a first surface configured to contact a first thermal stage            of the cryostat;        -   a second surface configured to contact a second thermal            stage of the cryostat, and        -   wherein the thermal linkage is operable to selectively move            between a first position, where the thermal linkage            simultaneously contacts the first thermal stage and the            second thermal stage and defines a thermal pathway            therebetween, and a second position, where the thermal            linkage is not contact with at least one of the first            thermal stage and the second thermal stage and the thermal            pathway is broken.-   Example 29. The heat switch of example 28, wherein the thermal    linkage is formed of a material having a thermal conductivity of at    least 1 W/(m·K) at 4 K.-   Example 30. The heat switch of example 29, wherein the material    comprises a cylindrical body.-   Example 31. The heat switch of example 29 or 30, wherein the    material comprises a braided body.-   Example 32. The heat switch of example 28 or any one of examples    29-31, wherein the thermal linkage is configured to move linearly    between the first position and the second position.-   Example 33. The heat switch of example 28 or any one of examples    29-32, wherein the thermal linkage is configured to rotate between    the first position and the second position.-   Example 34. The heat switch of example 28 or any one of examples    29-33, comprising:    -   an actuator coupled to the thermal linkage and operable to move        the thermal linkage between the first position and the second        position.-   Example 35. The heat switch of example 28 or any one of examples    29-33, comprising:    -   an actuator operable to move the thermal linkage between the        first position and the second position; and    -   a thermally-insulating element coupling the actuator to the        thermal linkage.-   Example 36. The heat switch of example 35, wherein the    thermally-insulating element is formed of a material having a    thermal conductivity less than 0.5 W/(m·K) at or below 50 K.-   Example 37. The heat switch of example 35, wherein the    thermally-insulating element comprises a cylindrical body.-   Example 38. The heat switch of example 35 or 36, wherein the    thermally-insulating element comprises a braided body.-   Example 39. The heat switch of example 28 or any one of examples    29-38, comprising:    -   the first thermal stage and the second thermal stage of the        cryostat;    -   wherein the first thermal stage is configured to operate at a        first temperature; and    -   wherein the second thermal stage is configured to operate at a        second temperature lower than the first temperature.-   Example 40. The heat switch of example 39, wherein the cryostat    comprises a spatial sequence of thermal stages and the first thermal    stage and the second thermal stage are adjacent each other in the    spatial sequence of thermal stages.-   Example 41. The heat switch of example 39, wherein the cryostat    comprises a spatial sequence of thermal stages and the first thermal    stage and the second thermal stage are separated by at least one    thermal stage in the spatial sequence of thermal stages.-   Example 42. The heat switch of example 39 or any one of examples    40-41, wherein the second temperature is no greater than 10 K.-   Example 43. The heat switch of example 39 or any one of examples    40-41, wherein the second temperature is no greater than 10 K and    the first temperature is no greater than 75 K.-   Example 44. The heat switch of example 39 or any one of examples    40-43, comprising:    -   a thermally-conductive element disposed between the first        surface of the thermal linkage and the first thermal stage or        disposed between the second surface of the thermal linkage and        the second thermal stage.-   Example 45. The heat switch of example 39 or any one of examples    40-43, comprising    -   a thermally-conductive element disposed between the first        surface of the thermal linkage and the first thermal stage or        disposed between the second surface of the thermal linkage and        the second thermal stage; and    -   wherein the thermally-conductive element is formed of a material        having a thermal conductivity of at least 1 W/(m·K) at 4 K.-   Example 46. A method for controlling a flow of heat between thermal    stages of a cryostat, the method comprising:    -   altering a first temperature of a first thermal stage of the        cryostat;    -   altering a second temperature of a second thermal stage of the        cryostat; and    -   selectively moving a thermal linkage between a first position,        where the thermal linkage simultaneously contacts the first        thermal stage and the second thermal stage and defines a thermal        pathway therebetween, and a second position, where the thermal        linkage is not contact with at least one of the first thermal        stage and the second thermal stage and the thermal pathway is        broken.-   Example 47. The method of example 46, wherein selectively moving the    thermal linkage comprises moving the thermal linkage linearly    between the first position and the second position.-   Example 48. The method of example 46 or 47, wherein selectively    moving the thermal linkage comprises rotating the thermal linkage    between the first position and the second position.-   Example 49. The method of example 46 or any one of examples 47-48,    -   wherein altering the first temperature of the first thermal        stage comprises cooling the first thermal stage;    -   wherein altering the second temperature of the second thermal        stage comprises cooling the second thermal stage; and    -   wherein selectively moving the thermal linkage comprises        transferring heat from the second thermal stage to the first        thermal stage when the thermal linkage is in the first position.-   Example 50. The method of example 49, wherein the second thermal    stage is cooled to a temperature no greater than 10 K.-   Example 51. The method of example 49, wherein the second thermal    stage is cooled to a temperature no greater than 10 K and the first    thermal stage is cooled to a temperature no greater than 75 K.-   Example 52. The method of example 46 or any one of examples 47-48,    -   wherein altering the first temperature of the first thermal        stage comprises heating the first thermal stage;    -   wherein altering the second temperature of the second thermal        stage comprises heating the second thermal stage; and    -   wherein selectively moving the thermal linkage comprises        transferring heat from the first thermal stage to the second        thermal stage when the thermal linkage is in the first position.-   Example 53. The method of example 52, wherein the second thermal    stage is heated from a temperature no greater than 10 K.-   Example 54. The method of example 52, wherein the second thermal    stage is heated from a temperature no greater than 10 K and the    first thermal stage is heated from a temperature no greater than 75    K.-   Example 55. The method of example 46 or any one of examples 47-54,    wherein the cryostat comprises a spatial sequence of thermal stages    and the first thermal stage and the second thermal stage are    adjacent each other in the spatial sequence of thermal stages.-   Example 56. The method of claim 46 or any one of examples 47-54,    wherein the cryostat comprises a spatial sequence of thermal stages    and the first thermal stage and the second thermal stage are    separated by at least one thermal stage in the spatial sequence of    thermal stages.

While this specification contains many details, these should not beunderstood as limitations on the scope of what may be claimed, butrather as descriptions of features specific to particular examples.Certain features that are described in this specification or shown inthe drawings in the context of separate implementations can also becombined. Conversely, various features that are described or shown inthe context of a single implementation can also be implemented inmultiple embodiments separately or in any suitable subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single product or packagedinto multiple products.

A number of embodiments have been described herein. Nevertheless, itwill be understood that various modifications can be made to theseembodiments. Accordingly, other embodiments are within the scope of thefollowing claims.

What is claimed is:
 1. A heat switch for a cryostat, comprising: athermal linkage, comprising: a first surface configured to contact afirst thermal stage of the cryostat; a second surface configured tocontact a second thermal stage of the cryostat, and wherein the thermallinkage is operable to selectively move between a first position, wherethe thermal linkage simultaneously contacts the first thermal stage andthe second thermal stage and defines a thermal pathway therebetween, anda second position, where the thermal linkage is not in contact with thefirst thermal stage and not in contact with the second thermal stage andthe thermal pathway is broken.
 2. The heat switch of claim 1, whereinthe thermal linkage is formed of a material having a thermalconductivity of at least 1 W/(m·K) at 4 K.
 3. The heat switch of claim1, wherein the thermal linkage is configured to move linearly betweenthe first position and the second position.
 4. The heat switch of claim1, wherein the thermal linkage is configured to rotate between the firstposition and the second position.
 5. The heat switch of claim 1,comprising: an actuator operable to move the thermal linkage between thefirst position and the second position; and a thermally-insulatingelement coupling the actuator to the thermal linkage.
 6. The heat switchof claim 5, wherein the thermally-insulating element is formed of amaterial having a thermal conductivity less than 0.5 W/(m·K) at or below50 K.
 7. The heat switch of claim 1, wherein the first thermal stage isconfigured to operate at a first temperature; and the second thermalstage is configured to operate at a second temperature lower than thefirst temperature.
 8. The heat switch of claim 7, wherein the cryostatcomprises a spatial sequence of thermal stages, and the first thermalstage and the second thermal stage are separated by at least one thermalstage in the spatial sequence of thermal stages.
 9. The heat switch ofclaim 7, wherein the second temperature is equal to or less than 10 K.10. The heat switch of claim 7, comprising one or both of: a firstthermally-conductive element disposed between the first surface of thethermal linkage and the first thermal stage; and a secondthermally-conductive element disposed between the second surface of thethermal linkage and the second thermal stage.
 11. A method forcontrolling a flow of heat between thermal stages of a cryostat, themethod comprising: altering a first temperature of a first thermal stageof the cryostat; altering a second temperature of a second thermal stageof the cryostat; and selectively moving a thermal linkage between afirst position, where the thermal linkage simultaneously contacts thefirst thermal stage and the second thermal stage and defines a thermalpathway therebetween, and a second position, where the thermal linkageis not in contact with the first thermal stage and not in contact withthe second thermal stage and the thermal pathway is broken.
 12. Themethod of claim 11, wherein selectively moving the thermal linkagecomprises moving the thermal linkage linearly between the first positionand the second position.
 13. The method of claim 11, wherein selectivelymoving the thermal linkage comprises rotating the thermal linkagebetween the first position and the second position.
 14. The method ofclaim 11, wherein altering the first temperature of the first thermalstage comprises cooling the first thermal stage; wherein altering thesecond temperature of the second thermal stage comprises cooling thesecond thermal stage; and wherein selectively moving the thermal linkagecomprises transferring heat from the second thermal stage to the firstthermal stage when the thermal linkage is in the first position.
 15. Themethod of claim 14, wherein the second thermal stage is cooled to atemperature equal to or less than 10 K.
 16. The method of claim 11,wherein altering the first temperature of the first thermal stagecomprises heating the first thermal stage; wherein altering the secondtemperature of the second thermal stage comprises heating the secondthermal stage; and wherein selectively moving the thermal linkagecomprises transferring heat from the first thermal stage to the secondthermal stage when the thermal linkage is in the first position.
 17. Themethod of claim 16, wherein the second thermal stage is heated from atemperature equal to or less than 10 K.
 18. The heat switch of claim 2,wherein the material comprises a cylindrical body.
 19. The heat switchof claim 2, wherein the material comprises a braided body.
 20. The heatswitch of claim 1, comprising: an actuator coupled to the thermallinkage and operable to move the thermal linkage between the firstposition and the second position.
 21. The heat switch of claim 5,wherein the thermally-insulating element comprises a cylindrical body.22. The heat switch of claim 5, wherein the thermally-insulating elementcomprises a braided body.
 23. The heat switch of claim 7, wherein thecryostat comprises a spatial sequence of thermal stages, and the firstthermal stage and the second thermal stage are adjacent to each other inthe spatial sequence of thermal stages.
 24. The heat switch of claim 7,wherein the second temperature is equal to or less than 10 K and thefirst temperature is equal to or less than 75 K.
 25. The heat switch ofclaim 7, comprising: a thermally-conductive element disposed between thefirst surface of the thermal linkage and the first thermal stage ordisposed between the second surface of the thermal linkage and thesecond thermal stage; and wherein the thermally-conductive element isformed of a material having a thermal conductivity of at least 1 W/(m·K)at 4 K.
 26. The method of claim 14, wherein the second thermal stage iscooled to a temperature equal to or less than 10 K and the first thermalstage is cooled to a temperature equal to or less than 75 K.
 27. Themethod of claim 16, wherein the second thermal stage is heated from atemperature equal to or less than 10 K and the first thermal stage isheated from a temperature equal to or less than 75 K.
 28. The method ofclaim 11, wherein the cryostat comprises a spatial sequence of thermalstages, and the first thermal stage and the second thermal stage areadjacent to each other in the spatial sequence of thermal stages. 29.The method of claim 11, wherein the cryostat comprises a spatialsequence of thermal stages, and the first thermal stage and the secondthermal stage are separated by at least one thermal stage in the spatialsequence of thermal stages.